I'm in a totally different situation from you, so I cannot tell you where to order from. I don't buy any treats for mine. I let them free range and find their own treats. I will give you a link to a treats chart in the Learning Center on this site. If you read it carefully, it will help you see what you can or should not give as treats.
https://www.backyardchickens.com/web/viewblog.php?id=2593-Treats_Chart
There is a section at the bottom that tells you what you should not feed your chickens. This does not mean that they will fall over dead the instant they take a bite of these things. Most of these "don't feed them" either do long term damage or are generally not in high enough concentration to do much damage. Let me give an example. Apple seeds contain cyanide, which is a poison. But one apple seed does not contain enough to do any damage. You can safely feed an apple to your chickens and not worry about them eating a seed or two. If they forage under an apple tree and eat apples that fall on the ground, they are not going to get enough apple seeds to do any damage. But if you make apple jelly or apple cider and have a bunch of the seeds left over, put that concentration of seeds where they cannot get to them. One of the things we love to give them and that they love, cabbage, contains a substance that can be harmful, but they would have to eat a tremendous amount of cabbage to do them any harm. So just give the treats in moderation and don't over-obsess about it.
Another thing I'll point out about the chart that seems to get misunderstood a lot. GREEN potato peelings contain a substance that is dangerous to humans and chickens. GREEN potato peelings, not the regular ones. Regular potato peelings do not contain this dangerous chemical and are fine. And the comment about beans is about uncooked or undercooked beans. Cooked beans are fine.
The feed store folks are right. That layer feed contains everything they need and in pretty balanced amounts. If you feed anything other than that, you take the chance of upsetting their balanced diet. We do that all the time. As long as you only give them what they can clean up in about 10 to 20 minutes, their diet is not going to become unbalanced enough to cause any problem at all. And it is nice to vary the treats you give them.
One last word of warning. Many people give grass trimmings, which they love and which does provide color to the yolks and some vitamins and minerals. But when chickens forage on grass trimmings, they bite off fairly small chunks and eat them. If you give them loose grass trimmings, they eat the whole thing because they cannot break it up. If they eat several really long blades of grass, weeds, or such, it can get balled up in their crop and cause an impacted crop. It does not happen each and every time, but it does happen. Usually, if you use a mulching lawn mower or you break the stuff up into pieces that are less than 2" long, it is not a problem. And don't give them a huge amount. Again, this is not something to obsess over, just something to be aware of.
I think if you look at the treats chart, you will find plenty of extra stuff in your kitchen or growing outside that you can give them without having to order or buy anything. Good luck! Hope this helps.